Rainfall Atlas of Hawaii

What is the Rainfall Atlas of Hawaiʻi?

The Hawaiian Islands have one of the most diverse rainfall patterns on earth. The mountainous terrain, persistent trade winds, heating and cooling of the land, and the regular presence of a stable atmospheric layer at an elevation of around 7,000 ft. interact to produce areas of uplift in distinct spatial patterns anchored to the topography. The resulting clouds and rainfall produced by this uplift lead to dramatic differences in mean rainfall over short distances. Knowledge of the mean rainfall patterns is critically important for a variety of resource management issues, including ground water and surface water development and protection, controlling and eradicating invasive species, protecting and restoring native ecosystems, and planning for the effects of global warming.

Be sure to check out the interactive map! It may need a few minutes to load on your first visit.

Updates: Seasonal Rainfall Trend Maps from 1920-2012 and 1983-2012 available for download (10/2018)Month-Year Rainfall Maps from 1920-2012 are available for download (08/2015)Raingage data (tabular) have been updated through 2012 (04/2015)
All Moloka‘i maps have been updated (07/2014)

The Rainfall Atlas of Hawaiʻi is a set of maps of the spatial patterns of rainfall for the major Hawaiian Islands. Maps are available for mean monthly and annual rainfall. The maps represent our best estimates of the mean rainfall for the 30-yr base period 1978–2007. However, for many reasons, it is not possible to determine the exact value of mean rainfall for any location. Therefore, for every map of mean rainfall, we provide a corresponding map of uncertainty. Uncertainty tends to be greatest where we have the poorest information about rainfall, for example in remote locations far from the nearest raingage.

This web site was developed to make the rainfall maps, data, and related information easily accessible. The maps depict rainfall patterns by color and/or by isohyets (lines of equal rainfall). The interactive map allows users to see the patterns of mean monthly and annual rainfall and corresponding uncertainty, zoom in on areas of particular interest, navigate to specific locations with the help of a choice of different base maps, and click on any location to get the mean annual rainfall and a graph and table of mean monthly rainfall. The locations of stations can also be shown on the interactive map. Clicking on a station gives both station and mapped estimates of monthly rainfall along with station metadata.

Rainfall maps can also be downloaded in various forms. Our analysis produced digital maps called rasters or grids. On these maps, the islands are divided into 8.1-arcsecond spatial units, or approximately 234 × 250 m (770 × 820 ft). Rainfall and uncertainty are estimated for each spatial unit. GIS (Geographic Information System) users can obtain mean and uncertainty maps as raster files. Alternatively, image files showing rainfall patterns by color and/or by isohyets can be downloaded.

Raingage in Haleakalā, Maui. Photo credit: John DeLay

Rainfall measurements taken at over 1,000 stations were used as the principal source of information in the development of the rainfall maps. Files containing estimated mean monthly and annual rainfall and uncertainty for each station used in the analysis are available for download. A file with information on each station, including the name, observer, location, elevation, and period of record, is also available.

This website is part of a family of websites providing data on the climate of Hawai‘i. The Rainfall Atlas of Hawai‘i covers only rainfall. The other three websites each provide data for many other variables including evapotranspiration, solar radiation, albedo, cloud frequency, temperature, relative humidity, etc. These other three websites contain the same variables, but each is presented with a particular focus.

Conditions of Use

All maps, data, graphs, and narrative material available on this web site are subject to the following conditions.

Images. All color and isohyet rainfall map images presented here are available for reproduction and distribution. The map source must be included in any reproduction.

Gridded Map and Tabular Data. All gridded and tabular data and all other data presented here are available for reproduction and distribution for non-commercial use. The data source must be prominently stated whenever presenting these data in any form or any product derived in whole or part from these data.

Crediting the Source. Any reproduction or use of the images, maps, graphs, data, other information, or any product derived in whole or part from these data must credit the source in the following manner:

Sponsors of the 2011 Rainfall Atlas of Hawaiʻi. The 2011 Rainfall Atlas of Hawai‘i was developed under an agreement between the State of Hawaiʻi Commission on Water Resource Management and the U.S. Army Corps of Engineers, Honolulu District under Section 22 of the Water Resources Development Act of 1974.

No Warranty. We have made every effort to make all images, maps, graphs, data, and other information provided on this web site accurate and error-free. However, we do not guarantee the accuracy of any images, maps, graphs, data, or other information. All content is provided without warranty of any kind.